1. Field of the Invention
This invention relates to a thermal transfer line printer that is suitably used for forming an intermediate transfer type image in which ink of a multi-color ink sheet is transferred on an intermediate transfer sheet by means of a line thermal head to form a primary image and the primary image is retransferred on a transfer medium to form an image on the transfer medium.
2. Description of the Related Art
Heretofore, the intermediate transfer type thermal transfer line printer for forming an image on a transfer medium by means of a method in which ink of a multi-color ink film is transferred on an intermediate transfer medium by use of a line thermal head to form a primary image and the primary image is retransferred on a transfer medium by use of a retransfer means has been used widely as an output apparatus of a computer or ward processor. The reasons are that a high quality printed image can be formed easily on various transfer media such as CD, CD-R, MO, DVD, and various cards in addition to regular paper and that the intermediate transfer type thermal transfer line printer is operated with low noise, available at low cost, and needs less maintenance.
A conventional thermal transfer line printer as described hereinabove operates as described hereunder. A line thermal head is brought into down-state in which the line thermal head is brought into contact with a platen roller with interposition of an ink sheet and an intermediate transfer sheet that are formed in the configuration of long sheet in this order in a primary image forming section. Heating elements of the line thermal head are heated selectively based on the printing information with feeding the ink sheet and the intermediate transfer sheet in this state to thereby melt or sublimate the ink carried on the ink sheet so that the ink is transferred on the intermediate transfer sheet, and a reverse image that is served as the intermediate image for one page, namely one picture, is formed on the intermediate transfer sheet. Thereafter, the intermediate transfer sheet is moved and the primary image formed on the intermediate transfer sheet is concomitantly moved to the position just before the retransfer section, and then the primary image is registered with a transfer medium with aid of the register mark formed on the intermediate transfer sheet. Then, the primary image formed on the intermediate transfer sheet is melted or sublimated by applying heat and pressure of retransfer means comprising a heating roller in a retransfer section, and the primary image is retransferred and fixed on the transfer medium to thereby form a desired image on the transfer medium.
At that time, in the case where a monochrome image of one color is formed on a transfer medium, one pass is enough for forming an image.
On the other hand, in the case where a multi-color image is formed on a transfer medium, a multi-color intermediate image is formed by means of so-called swing-back technique as described hereunder. A multi-color ink sheet on which a plurality of color ink regions are arranged so that different colors are repeated adjacently in the longitudinal direction issued as an ink sheet. At first, a reverse image of the first color ink that is carried on the multi-color ink sheet is formed on an intermediate transfer sheet. Then, a line thermal head is brought into head-up state in which the line thermal head is being separated from a platen, and the intermediate transfer sheet is moved reversely to the transfer preparation position in this state. The reverse image formed with the first color ink is returned to the transfer position for start alignment, and a reverse image of the next color is transferred on the reverse image of the first color one on the other.
In detail, in the case where a full-color image is formed, a multi-color ink sheet on which a plurality of ink regions, each of which consists of four colors, for example, K (black), Y (yellow), M (magenta), and C (cyan) are arranged so that the different colors are repeated adjacently in the longitudinal direction and which has color-discrimination marks on the boundary between different ink regions is used as an ink sheet.
More in detail, at first, a K-color reverse image for one page picture is formed on an intermediate transfer sheet by use of a K-color ink region on the multi-color ink sheet. Next, the intermediate transfer sheet that has been moved during the primary image forming operation is moved in the reverse direction to the transfer preparation position to align the K-color reverse image formed on the intermediate transfer sheet, and a Y-color ink region that is positioned adjacent to the K-color ink region of the multi-color ink sheet is aligned. With the use of the Y-color ink region in the multi-color sheet, a Y-color reverse image for one page picture is formed over the K-color reverse image for one page picture that has been formed on the intermediate transfer sheet. Similarly, reverse images of an M-color ink region and a C-color ink region are formed on the intermediate transfer sheet in this order to thereby form a full-color intermediate image for one page picture on the intermediate transfer sheet.
The conventional register mark that has been formed on an intermediate transfer sheet previously or is formed simultaneously when a primary image is formed on an intermediate transfer sheet.
Furthermore, the intermediate transfer sheet is wound between the intermediate transfer sheet feeding roller and the intermediate transfer sheet winding roller. The intermediate transfer sheet winding roller is rotated by means of the driving force of the intermediate transfer sheet feeding motor when a reverse image is formed, and the intermediate transfer sheet is sent out from the intermediate transfer sheet feeding roller and wound on the intermediate transfer sheet winding roller. Furthermore, in the case where the intermediate transfer sheet is moved back to the printing preparation position to form a full-color image, the intermediate transfer sheet feeding roller is rotated reversely by means of the driving force of the intermediate transfer sheet feeding motor. Thereby, the intermediate transfer sheet is moved in the reverse direction toward the intermediate transfer sheet feeding roller side from the intermediate transfer sheet winding roller side.
As the result of the above operation, the length of the intermediate transfer sheet wound on the intermediate transfer sheet feeding roller decreases with consumption of the intermediate transfer sheet to result in the reduced outside diameter of the intermediate transfer sheet feeding roller, and on the other hand the length of the intermediate transfer sheet wound on the intermediate transfer sheet winding roller increases to result in the increased outside diameter of the intermediate transfer sheet winding roller.
However, in the case of the abovementioned conventional thermal transfer line printer, the intermediate transfer sheet tension changes due to the change of the outside diameter of the intermediate transfer sheet feeding roller and the intermediate transfer sheet winding roller that is caused concomitantly with consumption of the intermediate transfer sheet, and the change causes the problem.
The change of the intermediate transfer sheet tension causes the change of moving length of the intermediate transfer sheet when the intermediate transfer sheet is moved reversely to the printing preparation position for forming a full-color image, and the change of the moving length causes positional deviation when the reverse image is transferred on the intermediate transfer sheet to result in the poor picture quality of the image printed on a transfer medium.
To solve the abovementioned problem, a thermal transfer line printer that is capable of maintaining the intermediate transfer sheet tension constant has been expected to be developed.
The present invention has been accomplished in view of the abovementioned problem, and it is the object of the present invention to provide a thermal transfer line printer that is capable of maintaining the intermediate transfer sheet tension constant with the simple structure.
To achieve the abovementioned object, a thermal transfer line printer in accordance with the present invention is characterized in that the thermal transfer line printer has a tension roller disposed so as to face to a moving path of the intermediate transfer sheet, a rotatable tension roller support frame that supports the tension roller rotatably and presses the tension roller against the intermediate transfer sheet, tension roller position detection means that detects a position of the tension roller, and a controller that controls operation of an intermediate transfer sheet feeding motor for feeding at least the intermediate transfer sheet, the tension roller position detection means is provided with a detector disposed on the tension roller support frame and a sensor that detects the position of the detector, and the controller, when feeding the intermediate transfer sheet, controls the intermediate transfer sheet feeding motor so as to maintain the detector in a detection range of the sensor.
At that time, the tension roller is provided preferably on any one of both sides of the primary image forming section, and more preferably the tension roller is provided on both sides of the primary image forming section.
By employing the abovementioned structure, the intermediate transfer sheet tension can be maintained constant with the simple structure.